docs: def _process_pub_dbxref(self, limit):
          """
          Xrefs for publications (ie FBrf = PMID)
          :param limit:
          :return:
  
          """
  
          if self.test_mode:
              graph = self.testgraph
          else:
              graph = self.graph
          model = Model(graph)
          raw = '/'.join((self.rawdir, 'pub_dbxref'))
          LOG.info("processing pub_dbxref")
  
          line_counter = 0
  
          with open(raw, 'r') as f:
              filereader = csv.reader(f, delimiter='\t', quotechar='\"')
              f.readline()  # read the header row; skip
              for line in filereader:
                  (pub_dbxref_id, pub_id, dbxref_id, is_current) = line
                  # 49648	43222	395730	t
  
                  pub_key = pub_id
                  pub_id = self.idhash['publication'][pub_key]
  
                  if self.test_mode and int(pub_key) not in self.test_keys['pub']:
                      continue
  
                  # get any dbxrefs for pubs, including pmids and dois
                  dbxref_key = dbxref_id
                  if str(dbxref_key) in self.dbxrefs:
                      dbxrefs = self.dbxrefs[str(dbxref_key)]
                      # pub_dbs = [75, 51, 76, 95, 126]
                      pmid_ids = [50, 77, 275, 286, 347]
                      # flybase_ids = [4]  # TODO unused
                      isbn = [75, 51]
                      for d in dbxrefs:
                          dbxref_id = None
                          if int(d) in pmid_ids:
                              if re.match(r'^PMID', dbxrefs[d]):
                                  dbxref_id = dbxrefs[d].strip()
                              else:
                                  dbxref_id = 'PMID:'+dbxrefs[d].strip()
                              model.makeLeader(dbxref_id)
                          elif int(d) in isbn:
                              dbxref_id = 'ISBN:'+dbxrefs[d].strip()
                          elif int(d) == 161:
                              dbxref_id = 'DOI:'+dbxrefs[d].strip()
                          # elif int(d) == 4:
                          #     dbxref_id = 'FlyBase:'+dbxrefs[d].strip()
  
                          if dbxref_id is not None:
                              reference = Reference(
                                  graph, dbxref_id, self.globaltt['publication'])
                              reference.addRefToGraph()
                              model.addSameIndividual(pub_id, dbxref_id)
                              line_counter += 1
  
                  if not self.test_mode and limit is not None and line_counter > limit:
                      break
  
          return